claqr5.f

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*> \brief \b CLAQR5 performs a single small-bulge multi-shift QR sweep.
*
*  =========== DOCUMENTATION ===========
*
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*            http://www.netlib.org/lapack/explore-html/
*
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*
*  Definition:
*  ===========
*
*       SUBROUTINE CLAQR5( WANTT, WANTZ, KACC22, N, KTOP, KBOT, NSHFTS, S,
*                          H, LDH, ILOZ, IHIZ, Z, LDZ, V, LDV, U, LDU, NV,
*                          WV, LDWV, NH, WH, LDWH )
*
*       .. Scalar Arguments ..
*       INTEGER            IHIZ, ILOZ, KACC22, KBOT, KTOP, LDH, LDU, LDV,
*      $                   LDWH, LDWV, LDZ, N, NH, NSHFTS, NV
*       LOGICAL            WANTT, WANTZ
*       ..
*       .. Array Arguments ..
*       COMPLEX            H( LDH, * ), S( * ), U( LDU, * ), V( LDV, * ),
*      $                   WH( LDWH, * ), WV( LDWV, * ), Z( LDZ, * )
*       ..
*
*
*> \par Purpose:
*  =============
*>
*> \verbatim
*>
*>    CLAQR5 called by CLAQR0 performs a
*>    single small-bulge multi-shift QR sweep.
*> \endverbatim
*
*  Arguments:
*  ==========
*
*> \param[in] WANTT
*> \verbatim
*>          WANTT is LOGICAL
*>             WANTT = .true. if the triangular Schur factor
*>             is being computed.  WANTT is set to .false. otherwise.
*> \endverbatim
*>
*> \param[in] WANTZ
*> \verbatim
*>          WANTZ is LOGICAL
*>             WANTZ = .true. if the unitary Schur factor is being
*>             computed.  WANTZ is set to .false. otherwise.
*> \endverbatim
*>
*> \param[in] KACC22
*> \verbatim
*>          KACC22 is INTEGER with value 0, 1, or 2.
*>             Specifies the computation mode of far-from-diagonal
*>             orthogonal updates.
*>        = 0: CLAQR5 does not accumulate reflections and does not
*>             use matrix-matrix multiply to update far-from-diagonal
*>             matrix entries.
*>        = 1: CLAQR5 accumulates reflections and uses matrix-matrix
*>             multiply to update the far-from-diagonal matrix entries.
*>        = 2: Same as KACC22 = 1. This option used to enable exploiting
*>             the 2-by-2 structure during matrix multiplications, but
*>             this is no longer supported.
*> \endverbatim
*>
*> \param[in] N
*> \verbatim
*>          N is INTEGER
*>             N is the order of the Hessenberg matrix H upon which this
*>             subroutine operates.
*> \endverbatim
*>
*> \param[in] KTOP
*> \verbatim
*>          KTOP is INTEGER
*> \endverbatim
*>
*> \param[in] KBOT
*> \verbatim
*>          KBOT is INTEGER
*>             These are the first and last rows and columns of an
*>             isolated diagonal block upon which the QR sweep is to be
*>             applied. It is assumed without a check that
*>                       either KTOP = 1  or   H(KTOP,KTOP-1) = 0
*>             and
*>                       either KBOT = N  or   H(KBOT+1,KBOT) = 0.
*> \endverbatim
*>
*> \param[in] NSHFTS
*> \verbatim
*>          NSHFTS is INTEGER
*>             NSHFTS gives the number of simultaneous shifts.  NSHFTS
*>             must be positive and even.
*> \endverbatim
*>
*> \param[in,out] S
*> \verbatim
*>          S is COMPLEX array, dimension (NSHFTS)
*>             S contains the shifts of origin that define the multi-
*>             shift QR sweep.  On output S may be reordered.
*> \endverbatim
*>
*> \param[in,out] H
*> \verbatim
*>          H is COMPLEX array, dimension (LDH,N)
*>             On input H contains a Hessenberg matrix.  On output a
*>             multi-shift QR sweep with shifts SR(J)+i*SI(J) is applied
*>             to the isolated diagonal block in rows and columns KTOP
*>             through KBOT.
*> \endverbatim
*>
*> \param[in] LDH
*> \verbatim
*>          LDH is INTEGER
*>             LDH is the leading dimension of H just as declared in the
*>             calling procedure.  LDH >= MAX(1,N).
*> \endverbatim
*>
*> \param[in] ILOZ
*> \verbatim
*>          ILOZ is INTEGER
*> \endverbatim
*>
*> \param[in] IHIZ
*> \verbatim
*>          IHIZ is INTEGER
*>             Specify the rows of Z to which transformations must be
*>             applied if WANTZ is .TRUE.. 1 <= ILOZ <= IHIZ <= N
*> \endverbatim
*>
*> \param[in,out] Z
*> \verbatim
*>          Z is COMPLEX array, dimension (LDZ,IHIZ)
*>             If WANTZ = .TRUE., then the QR Sweep unitary
*>             similarity transformation is accumulated into
*>             Z(ILOZ:IHIZ,ILOZ:IHIZ) from the right.
*>             If WANTZ = .FALSE., then Z is unreferenced.
*> \endverbatim
*>
*> \param[in] LDZ
*> \verbatim
*>          LDZ is INTEGER
*>             LDA is the leading dimension of Z just as declared in
*>             the calling procedure. LDZ >= N.
*> \endverbatim
*>
*> \param[out] V
*> \verbatim
*>          V is COMPLEX array, dimension (LDV,NSHFTS/2)
*> \endverbatim
*>
*> \param[in] LDV
*> \verbatim
*>          LDV is INTEGER
*>             LDV is the leading dimension of V as declared in the
*>             calling procedure.  LDV >= 3.
*> \endverbatim
*>
*> \param[out] U
*> \verbatim
*>          U is COMPLEX array, dimension (LDU,2*NSHFTS)
*> \endverbatim
*>
*> \param[in] LDU
*> \verbatim
*>          LDU is INTEGER
*>             LDU is the leading dimension of U just as declared in the
*>             in the calling subroutine.  LDU >= 2*NSHFTS.
*> \endverbatim
*>
*> \param[in] NV
*> \verbatim
*>          NV is INTEGER
*>             NV is the number of rows in WV agailable for workspace.
*>             NV >= 1.
*> \endverbatim
*>
*> \param[out] WV
*> \verbatim
*>          WV is COMPLEX array, dimension (LDWV,2*NSHFTS)
*> \endverbatim
*>
*> \param[in] LDWV
*> \verbatim
*>          LDWV is INTEGER
*>             LDWV is the leading dimension of WV as declared in the
*>             in the calling subroutine.  LDWV >= NV.
*> \endverbatim
*
*> \param[in] NH
*> \verbatim
*>          NH is INTEGER
*>             NH is the number of columns in array WH available for
*>             workspace. NH >= 1.
*> \endverbatim
*>
*> \param[out] WH
*> \verbatim
*>          WH is COMPLEX array, dimension (LDWH,NH)
*> \endverbatim
*>
*> \param[in] LDWH
*> \verbatim
*>          LDWH is INTEGER
*>             Leading dimension of WH just as declared in the
*>             calling procedure.  LDWH >= 2*NSHFTS.
*> \endverbatim
*>
*  Authors:
*  ========
*
*> \author Univ. of Tennessee
*> \author Univ. of California Berkeley
*> \author Univ. of Colorado Denver
*> \author NAG Ltd.
*
*> \ingroup complexOTHERauxiliary
*
*> \par Contributors:
*  ==================
*>
*>       Karen Braman and Ralph Byers, Department of Mathematics,
*>       University of Kansas, USA
*>
*>       Lars Karlsson, Daniel Kressner, and Bruno Lang
*>
*>       Thijs Steel, Department of Computer science,
*>       KU Leuven, Belgium
*
*> \par References:
*  ================
*>
*>       K. Braman, R. Byers and R. Mathias, The Multi-Shift QR
*>       Algorithm Part I: Maintaining Well Focused Shifts, and Level 3
*>       Performance, SIAM Journal of Matrix Analysis, volume 23, pages
*>       929--947, 2002.
*>
*>       Lars Karlsson, Daniel Kressner, and Bruno Lang, Optimally packed
*>       chains of bulges in multishift QR algorithms.
*>       ACM Trans. Math. Softw. 40, 2, Article 12 (February 2014).
*>
*  =====================================================================
      SUBROUTINE claqr5( WANTT, WANTZ, KACC22, N, KTOP, KBOT, NSHFTS, S,
     $                   H, LDH, ILOZ, IHIZ, Z, LDZ, V, LDV, U, LDU, NV,
     $                   WV, LDWV, NH, WH, LDWH )
      IMPLICIT NONE
*
*  -- LAPACK auxiliary routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      INTEGER            IHIZ, ILOZ, KACC22, KBOT, KTOP, LDH, LDU, LDV,
     $                   LDWH, LDWV, LDZ, N, NH, NSHFTS, NV
      LOGICAL            WANTT, WANTZ
*     ..
*     .. Array Arguments ..
      COMPLEX            H( LDH, * ), S( * ), U( LDU, * ), V( LDV, * ),
     $                   WH( LDWH, * ), WV( LDWV, * ), Z( LDZ, * )
*     ..
*
*  ================================================================
*     .. Parameters ..
      COMPLEX            ZERO, ONE
      PARAMETER          ( ZERO = ( 0.0e0, 0.0e0 ),
     $                   one = ( 1.0e0, 0.0e0 ) )
      REAL               RZERO, RONE
      PARAMETER          ( RZERO = 0.0e0, rone = 1.0e0 )
*     ..
*     .. Local Scalars ..
      COMPLEX            ALPHA, BETA, CDUM, REFSUM
      REAL               H11, H12, H21, H22, SAFMAX, SAFMIN, SCL,
     $                   smlnum, tst1, tst2, ulp
      INTEGER            I2, I4, INCOL, J, JBOT, JCOL, JLEN,
     $                   JROW, JTOP, K, K1, KDU, KMS, KRCOL,
     $                   m, m22, mbot, mtop, nbmps, ndcol,
     $                   ns, nu
      LOGICAL            ACCUM, BMP22
*     ..
*     .. External Functions ..
      REAL               SLAMCH
      EXTERNAL           SLAMCH
*     ..
*     .. Intrinsic Functions ..
*
      INTRINSIC          abs, aimag, conjg, max, min, mod, real
*     ..
*     .. Local Arrays ..
      COMPLEX            VT( 3 )
*     ..
*     .. External Subroutines ..
      EXTERNAL           cgemm, clacpy, claqr1, clarfg, claset, ctrmm,
     $                   slabad
*     ..
*     .. Statement Functions ..
      REAL               CABS1
*     ..
*     .. Statement Function definitions ..
      cabs1( cdum ) = abs( real( cdum ) ) + abs( aimag( cdum ) )
*     ..
*     .. Executable Statements ..
*
*     ==== If there are no shifts, then there is nothing to do. ====
*
      IF( nshfts.LT.2 )
     $   RETURN
*
*     ==== If the active block is empty or 1-by-1, then there
*     .    is nothing to do. ====
*
      IF( ktop.GE.kbot )
     $   RETURN
*
*     ==== NSHFTS is supposed to be even, but if it is odd,
*     .    then simply reduce it by one.  ====
*
      ns = nshfts - mod( nshfts, 2 )
*
*     ==== Machine constants for deflation ====
*
      safmin = slamch( 'SAFE MINIMUM' )
      safmax = rone / safmin
      CALL slabad( safmin, safmax )
      ulp = slamch( 'PRECISION' )
      smlnum = safmin*( real( n ) / ulp )
*
*     ==== Use accumulated reflections to update far-from-diagonal
*     .    entries ? ====
*
      accum = ( kacc22.EQ.1 ) .OR. ( kacc22.EQ.2 )
*
*     ==== clear trash ====
*
      IF( ktop+2.LE.kbot )
     $   h( ktop+2, ktop ) = zero
*
*     ==== NBMPS = number of 2-shift bulges in the chain ====
*
      nbmps = ns / 2
*
*     ==== KDU = width of slab ====
*
      kdu = 4*nbmps
*
*     ==== Create and chase chains of NBMPS bulges ====
*
      DO 180 incol = ktop - 2*nbmps + 1, kbot - 2, 2*nbmps
*
*        JTOP = Index from which updates from the right start.
*
         IF( accum ) THEN
            jtop = max( ktop, incol )
         ELSE IF( wantt ) THEN
            jtop = 1
         ELSE
            jtop = ktop
         END IF
*
         ndcol = incol + kdu
         IF( accum )
     $      CALL claset( 'ALL', kdu, kdu, zero, one, u, ldu )
*
*        ==== Near-the-diagonal bulge chase.  The following loop
*        .    performs the near-the-diagonal part of a small bulge
*        .    multi-shift QR sweep.  Each 4*NBMPS column diagonal
*        .    chunk extends from column INCOL to column NDCOL
*        .    (including both column INCOL and column NDCOL). The
*        .    following loop chases a 2*NBMPS+1 column long chain of
*        .    NBMPS bulges 2*NBMPS columns to the right.  (INCOL
*        .    may be less than KTOP and and NDCOL may be greater than
*        .    KBOT indicating phantom columns from which to chase
*        .    bulges before they are actually introduced or to which
*        .    to chase bulges beyond column KBOT.)  ====
*
         DO 145 krcol = incol, min( incol+2*nbmps-1, kbot-2 )
*
*           ==== Bulges number MTOP to MBOT are active double implicit
*           .    shift bulges.  There may or may not also be small
*           .    2-by-2 bulge, if there is room.  The inactive bulges
*           .    (if any) must wait until the active bulges have moved
*           .    down the diagonal to make room.  The phantom matrix
*           .    paradigm described above helps keep track.  ====
*
            mtop = max( 1, ( ktop-krcol ) / 2+1 )
            mbot = min( nbmps, ( kbot-krcol-1 ) / 2 )
            m22 = mbot + 1
            bmp22 = ( mbot.LT.nbmps ) .AND. ( krcol+2*( m22-1 ) ).EQ.
     $              ( kbot-2 )
*
*           ==== Generate reflections to chase the chain right
*           .    one column.  (The minimum value of K is KTOP-1.) ====
*
            IF ( bmp22 ) THEN
*
*              ==== Special case: 2-by-2 reflection at bottom treated
*              .    separately ====
*
               k = krcol + 2*( m22-1 )
               IF( k.EQ.ktop-1 ) THEN
                  CALL claqr1( 2, h( k+1, k+1 ), ldh, s( 2*m22-1 ),
     $                         s( 2*m22 ), v( 1, m22 ) )
                  beta = v( 1, m22 )
                  CALL clarfg( 2, beta, v( 2, m22 ), 1, v( 1, m22 ) )
               ELSE
                  beta = h( k+1, k )
                  v( 2, m22 ) = h( k+2, k )
                  CALL clarfg( 2, beta, v( 2, m22 ), 1, v( 1, m22 ) )
                  h( k+1, k ) = beta
                  h( k+2, k ) = zero
               END IF
 
*
*              ==== Perform update from right within 
*              .    computational window. ====
*
               DO 30 j = jtop, min( kbot, k+3 )
                  refsum = v( 1, m22 )*( h( j, k+1 )+v( 2, m22 )*
     $                     h( j, k+2 ) )
                  h( j, k+1 ) = h( j, k+1 ) - refsum
                  h( j, k+2 ) = h( j, k+2 ) -
     $                          refsum*conjg( v( 2, m22 ) )
   30          CONTINUE
*
*              ==== Perform update from left within 
*              .    computational window. ====
*
               IF( accum ) THEN
                  jbot = min( ndcol, kbot )
               ELSE IF( wantt ) THEN
                  jbot = n
               ELSE
                  jbot = kbot
               END IF
               DO 40 j = k+1, jbot
                  refsum = conjg( v( 1, m22 ) )*
     $                     ( h( k+1, j )+conjg( v( 2, m22 ) )*
     $                     h( k+2, j ) )
                  h( k+1, j ) = h( k+1, j ) - refsum
                  h( k+2, j ) = h( k+2, j ) - refsum*v( 2, m22 )
   40          CONTINUE
*
*              ==== The following convergence test requires that
*              .    the tradition small-compared-to-nearby-diagonals
*              .    criterion and the Ahues & Tisseur (LAWN 122, 1997)
*              .    criteria both be satisfied.  The latter improves
*              .    accuracy in some examples. Falling back on an
*              .    alternate convergence criterion when TST1 or TST2
*              .    is zero (as done here) is traditional but probably
*              .    unnecessary. ====
*
               IF( k.GE.ktop) THEN
                  IF( h( k+1, k ).NE.zero ) THEN
                     tst1 = cabs1( h( k, k ) ) + cabs1( h( k+1, k+1 ) )
                     IF( tst1.EQ.rzero ) THEN
                        IF( k.GE.ktop+1 )
     $                     tst1 = tst1 + cabs1( h( k, k-1 ) )
                        IF( k.GE.ktop+2 )
     $                     tst1 = tst1 + cabs1( h( k, k-2 ) )
                        IF( k.GE.ktop+3 )
     $                     tst1 = tst1 + cabs1( h( k, k-3 ) )
                        IF( k.LE.kbot-2 )
     $                     tst1 = tst1 + cabs1( h( k+2, k+1 ) )
                        IF( k.LE.kbot-3 )
     $                     tst1 = tst1 + cabs1( h( k+3, k+1 ) )
                        IF( k.LE.kbot-4 )
     $                     tst1 = tst1 + cabs1( h( k+4, k+1 ) )
                     END IF
                     IF( cabs1( h( k+1, k ) )
     $                   .LE.max( smlnum, ulp*tst1 ) ) THEN
                        h12 = max( cabs1( h( k+1, k ) ),
     $                        cabs1( h( k, k+1 ) ) )
                        h21 = min( cabs1( h( k+1, k ) ),
     $                        cabs1( h( k, k+1 ) ) )
                        h11 = max( cabs1( h( k+1, k+1 ) ),
     $                        cabs1( h( k, k )-h( k+1, k+1 ) ) )
                        h22 = min( cabs1( h( k+1, k+1 ) ),
     $                        cabs1( h( k, k )-h( k+1, k+1 ) ) )
                        scl = h11 + h12
                        tst2 = h22*( h11 / scl )
*
                        IF( tst2.EQ.rzero .OR. h21*( h12 / scl ).LE.
     $                      max( smlnum, ulp*tst2 ) )h( k+1, k ) = zero
                     END IF
                  END IF
               END IF
*
*              ==== Accumulate orthogonal transformations. ====
*
               IF( accum ) THEN
                  kms = k - incol
                  DO 50 j = max( 1, ktop-incol ), kdu
                     refsum = v( 1, m22 )*( u( j, kms+1 )+
     $                        v( 2, m22 )*u( j, kms+2 ) )
                     u( j, kms+1 ) = u( j, kms+1 ) - refsum
                     u( j, kms+2 ) = u( j, kms+2 ) -
     $                               refsum*conjg( v( 2, m22 ) )
  50                 CONTINUE
               ELSE IF( wantz ) THEN
                  DO 60 j = iloz, ihiz
                     refsum = v( 1, m22 )*( z( j, k+1 )+v( 2, m22 )*
     $                        z( j, k+2 ) )
                     z( j, k+1 ) = z( j, k+1 ) - refsum
                     z( j, k+2 ) = z( j, k+2 ) -
     $                             refsum*conjg( v( 2, m22 ) )
  60              CONTINUE
               END IF
            END IF
*
*           ==== Normal case: Chain of 3-by-3 reflections ====
*
            DO 80 m = mbot, mtop, -1
               k = krcol + 2*( m-1 )
               IF( k.EQ.ktop-1 ) THEN
                  CALL claqr1( 3, h( ktop, ktop ), ldh, s( 2*m-1 ),
     $                         s( 2*m ), v( 1, m ) )
                  alpha = v( 1, m )
                  CALL clarfg( 3, alpha, v( 2, m ), 1, v( 1, m ) )
               ELSE
*
*                 ==== Perform delayed transformation of row below
*                 .    Mth bulge. Exploit fact that first two elements
*                 .    of row are actually zero. ====
*
                  refsum = v( 1, m )*v( 3, m )*h( k+3, k+2 )
                  h( k+3, k   ) = -refsum
                  h( k+3, k+1 ) = -refsum*conjg( v( 2, m ) )
                  h( k+3, k+2 ) = h( k+3, k+2 ) -
     $                            refsum*conjg( v( 3, m ) )
*
*                 ==== Calculate reflection to move
*                 .    Mth bulge one step. ====
*
                  beta      = h( k+1, k )
                  v( 2, m ) = h( k+2, k )
                  v( 3, m ) = h( k+3, k )
                  CALL clarfg( 3, beta, v( 2, m ), 1, v( 1, m ) )
*
*                 ==== A Bulge may collapse because of vigilant
*                 .    deflation or destructive underflow.  In the
*                 .    underflow case, try the two-small-subdiagonals
*                 .    trick to try to reinflate the bulge.  ====
*
                  IF( h( k+3, k ).NE.zero .OR. h( k+3, k+1 ).NE.
     $                zero .OR. h( k+3, k+2 ).EQ.zero ) THEN
*
*                    ==== Typical case: not collapsed (yet). ====
*
                     h( k+1, k ) = beta
                     h( k+2, k ) = zero
                     h( k+3, k ) = zero
                  ELSE
*
*                    ==== Atypical case: collapsed.  Attempt to
*                    .    reintroduce ignoring H(K+1,K) and H(K+2,K).
*                    .    If the fill resulting from the new
*                    .    reflector is too large, then abandon it.
*                    .    Otherwise, use the new one. ====
*
                     CALL claqr1( 3, h( k+1, k+1 ), ldh, s( 2*m-1 ),
     $                            s( 2*m ), vt )
                     alpha = vt( 1 )
                     CALL clarfg( 3, alpha, vt( 2 ), 1, vt( 1 ) )
                     refsum = conjg( vt( 1 ) )*
     $                        ( h( k+1, k )+conjg( vt( 2 ) )*
     $                        h( k+2, k ) )
*
                     IF( cabs1( h( k+2, k )-refsum*vt( 2 ) )+
     $                   cabs1( refsum*vt( 3 ) ).GT.ulp*
     $                   ( cabs1( h( k, k ) )+cabs1( h( k+1,
     $                   k+1 ) )+cabs1( h( k+2, k+2 ) ) ) ) THEN
*
*                       ==== Starting a new bulge here would
*                       .    create non-negligible fill.  Use
*                       .    the old one with trepidation. ====
*
                        h( k+1, k ) = beta
                        h( k+2, k ) = zero
                        h( k+3, k ) = zero
                     ELSE
*
*                       ==== Starting a new bulge here would
*                       .    create only negligible fill.
*                       .    Replace the old reflector with
*                       .    the new one. ====
*
                        h( k+1, k ) = h( k+1, k ) - refsum
                        h( k+2, k ) = zero
                        h( k+3, k ) = zero
                        v( 1, m ) = vt( 1 )
                        v( 2, m ) = vt( 2 )
                        v( 3, m ) = vt( 3 )
                     END IF
                  END IF
               END IF
*
*              ====  Apply reflection from the right and
*              .     the first column of update from the left.
*              .     These updates are required for the vigilant
*              .     deflation check. We still delay most of the
*              .     updates from the left for efficiency. ====
*
               DO 70 j = jtop, min( kbot, k+3 )
                  refsum = v( 1, m )*( h( j, k+1 )+v( 2, m )*
     $                     h( j, k+2 )+v( 3, m )*h( j, k+3 ) )
                  h( j, k+1 ) = h( j, k+1 ) - refsum
                  h( j, k+2 ) = h( j, k+2 ) -
     $                          refsum*conjg( v( 2, m ) )
                  h( j, k+3 ) = h( j, k+3 ) -
     $                          refsum*conjg( v( 3, m ) )
   70          CONTINUE
*
*              ==== Perform update from left for subsequent
*              .    column. ====
*
               refsum =  conjg( v( 1, m ) )*( h( k+1, k+1 )
     $                  +conjg( v( 2, m ) )*h( k+2, k+1 )
     $                  +conjg( v( 3, m ) )*h( k+3, k+1 ) )
               h( k+1, k+1 ) = h( k+1, k+1 ) - refsum
               h( k+2, k+1 ) = h( k+2, k+1 ) - refsum*v( 2, m )
               h( k+3, k+1 ) = h( k+3, k+1 ) - refsum*v( 3, m )
*
*              ==== The following convergence test requires that
*              .    the tradition small-compared-to-nearby-diagonals
*              .    criterion and the Ahues & Tisseur (LAWN 122, 1997)
*              .    criteria both be satisfied.  The latter improves
*              .    accuracy in some examples. Falling back on an
*              .    alternate convergence criterion when TST1 or TST2
*              .    is zero (as done here) is traditional but probably
*              .    unnecessary. ====
*
               IF( k.LT.ktop)
     $              cycle
               IF( h( k+1, k ).NE.zero ) THEN
                  tst1 = cabs1( h( k, k ) ) + cabs1( h( k+1, k+1 ) )
                  IF( tst1.EQ.rzero ) THEN
                     IF( k.GE.ktop+1 )
     $                  tst1 = tst1 + cabs1( h( k, k-1 ) )
                     IF( k.GE.ktop+2 )
     $                  tst1 = tst1 + cabs1( h( k, k-2 ) )
                     IF( k.GE.ktop+3 )
     $                  tst1 = tst1 + cabs1( h( k, k-3 ) )
                     IF( k.LE.kbot-2 )
     $                  tst1 = tst1 + cabs1( h( k+2, k+1 ) )
                     IF( k.LE.kbot-3 )
     $                  tst1 = tst1 + cabs1( h( k+3, k+1 ) )
                     IF( k.LE.kbot-4 )
     $                  tst1 = tst1 + cabs1( h( k+4, k+1 ) )
                  END IF
                  IF( cabs1( h( k+1, k ) ).LE.max( smlnum, ulp*tst1 ) )
     $                 THEN
                     h12 = max( cabs1( h( k+1, k ) ),
     $                     cabs1( h( k, k+1 ) ) )
                     h21 = min( cabs1( h( k+1, k ) ),
     $                     cabs1( h( k, k+1 ) ) )
                     h11 = max( cabs1( h( k+1, k+1 ) ),
     $                     cabs1( h( k, k )-h( k+1, k+1 ) ) )
                     h22 = min( cabs1( h( k+1, k+1 ) ),
     $                     cabs1( h( k, k )-h( k+1, k+1 ) ) )
                     scl = h11 + h12
                     tst2 = h22*( h11 / scl )
*
                     IF( tst2.EQ.rzero .OR. h21*( h12 / scl ).LE.
     $                   max( smlnum, ulp*tst2 ) )h( k+1, k ) = zero
                  END IF
               END IF
   80       CONTINUE
*
*           ==== Multiply H by reflections from the left ====
*
            IF( accum ) THEN
               jbot = min( ndcol, kbot )
            ELSE IF( wantt ) THEN
               jbot = n
            ELSE
               jbot = kbot
            END IF
*
            DO 100 m = mbot, mtop, -1
               k = krcol + 2*( m-1 )
               DO 90 j = max( ktop, krcol + 2*m ), jbot
                  refsum = conjg( v( 1, m ) )*
     $                     ( h( k+1, j )+conjg( v( 2, m ) )*
     $                     h( k+2, j )+conjg( v( 3, m ) )*h( k+3, j ) )
                  h( k+1, j ) = h( k+1, j ) - refsum
                  h( k+2, j ) = h( k+2, j ) - refsum*v( 2, m )
                  h( k+3, j ) = h( k+3, j ) - refsum*v( 3, m )
   90          CONTINUE
  100       CONTINUE
*
*           ==== Accumulate orthogonal transformations. ====
*
            IF( accum ) THEN
*
*              ==== Accumulate U. (If needed, update Z later
*              .    with an efficient matrix-matrix
*              .    multiply.) ====
*
               DO 120 m = mbot, mtop, -1
                  k = krcol + 2*( m-1 )
                  kms = k - incol
                  i2 = max( 1, ktop-incol )
                  i2 = max( i2, kms-(krcol-incol)+1 )
                  i4 = min( kdu, krcol + 2*( mbot-1 ) - incol + 5 )
                  DO 110 j = i2, i4
                     refsum = v( 1, m )*( u( j, kms+1 )+v( 2, m )*
     $                        u( j, kms+2 )+v( 3, m )*u( j, kms+3 ) )
                     u( j, kms+1 ) = u( j, kms+1 ) - refsum
                     u( j, kms+2 ) = u( j, kms+2 ) -
     $                               refsum*conjg( v( 2, m ) )
                     u( j, kms+3 ) = u( j, kms+3 ) -
     $                               refsum*conjg( v( 3, m ) )
  110             CONTINUE
  120          CONTINUE
            ELSE IF( wantz ) THEN
*
*              ==== U is not accumulated, so update Z
*              .    now by multiplying by reflections
*              .    from the right. ====
*
               DO 140 m = mbot, mtop, -1
                  k = krcol + 2*( m-1 )
                  DO 130 j = iloz, ihiz
                     refsum = v( 1, m )*( z( j, k+1 )+v( 2, m )*
     $                        z( j, k+2 )+v( 3, m )*z( j, k+3 ) )
                     z( j, k+1 ) = z( j, k+1 ) - refsum
                     z( j, k+2 ) = z( j, k+2 ) -
     $                             refsum*conjg( v( 2, m ) )
                     z( j, k+3 ) = z( j, k+3 ) -
     $                             refsum*conjg( v( 3, m ) )
  130             CONTINUE
  140          CONTINUE
            END IF
*
*           ==== End of near-the-diagonal bulge chase. ====
*
  145    CONTINUE
*
*        ==== Use U (if accumulated) to update far-from-diagonal
*        .    entries in H.  If required, use U to update Z as
*        .    well. ====
*
         IF( accum ) THEN
            IF( wantt ) THEN
               jtop = 1
               jbot = n
            ELSE
               jtop = ktop
               jbot = kbot
            END IF
            k1 = max( 1, ktop-incol )
            nu = ( kdu-max( 0, ndcol-kbot ) ) - k1 + 1
*
*           ==== Horizontal Multiply ====
*
            DO 150 jcol = min( ndcol, kbot ) + 1, jbot, nh
               jlen = min( nh, jbot-jcol+1 )
               CALL cgemm( 'C', 'N', nu, jlen, nu, one, u( k1, k1 ),
     $                     ldu, h( incol+k1, jcol ), ldh, zero, wh,
     $                     ldwh )
               CALL clacpy( 'ALL', nu, jlen, wh, ldwh,
     $                      h( incol+k1, jcol ), ldh )
  150       CONTINUE
*
*           ==== Vertical multiply ====
*
            DO 160 jrow = jtop, max( ktop, incol ) - 1, nv
               jlen = min( nv, max( ktop, incol )-jrow )
               CALL cgemm( 'N', 'N', jlen, nu, nu, one,
     $                     h( jrow, incol+k1 ), ldh, u( k1, k1 ),
     $                     ldu, zero, wv, ldwv )
               CALL clacpy( 'ALL', jlen, nu, wv, ldwv,
     $                      h( jrow, incol+k1 ), ldh )
  160       CONTINUE
*
*           ==== Z multiply (also vertical) ====
*
            IF( wantz ) THEN
               DO 170 jrow = iloz, ihiz, nv
                  jlen = min( nv, ihiz-jrow+1 )
                  CALL cgemm( 'N', 'N', jlen, nu, nu, one,
     $                        z( jrow, incol+k1 ), ldz, u( k1, k1 ),
     $                        ldu, zero, wv, ldwv )
                  CALL clacpy( 'ALL', jlen, nu, wv, ldwv,
     $                         z( jrow, incol+k1 ), ldz )
  170          CONTINUE
            END IF
         END IF
  180 CONTINUE
*
*     ==== End of CLAQR5 ====
*
      END